The fact that is based on a tube, will it alter the signal in any way that will effect the measurements' accuracy? I mean, the tube gives a warmer sound, isnt this translated in an electrical point of vew somehow ?

Without getting into a whole diatribe, it boils down to this: for measurement purposes, there is no reason whatever to use tubes and a lot of reasons not to. An opamp-based mike preamp will be much superior in terms of reliability, stability, and accuracy.

__________________
“The short explanation is always dull. It generally includes the word ‘just.’ The explanation only becomes beautiful when you immerse yourself in every nuance.”